This invention relates generally to digital communication systems and particularly to apparatus and methods for generating digital signals. Still more particularly, this invention relates to fiber optic systems for generating digital words for use in digital signal processing systems.
The internal operations of digital computers and signal processing systems are in binary arithmetic. Such systems store and access information in units called words. A typical word size is 32 binary digits, or bits, although word sizes in current use range from 16 to 60 bits.
Realization of the full potential of single mode fiber optic systems in digital communications and data processing systems requires an efficient means of generating digital words and transferring the digital words to other components of the system. It is possible to produce a word generator by digitally weighting the outputs of selected taps of a tapped delay line.
A tapped delay line may be used as a means for generating a precisely timed series of pulses. The precision with which the pulses are timed depends upon the accuracy in locating the taps. In optical fiber, the propagation velocity is so great, being approximately 200,000 km/sec, that if the tap location is known to within 5 mm, the time delay from a specified reference point is known to within 25 ps. While the same is true of electrical delay lines, losses at frequencies of several gigahertz and above are extremely high and dependent upon the modulation frequency, whereas in optical fibers the losses are low and are independent of the modulation frequency.
In communication and control systems, delay lines are used to store signals for predetermined delay time periods and to supply the signals at output points at the end of the delay period. A tapped delay line is a variation of a delay line in which the signal is supplied at several different output points. Tapped delay line applications include use in pulse generators, correlators, and convoluting devices.
There are three basic types of tapped delay lines which may be used in the construction of a digital word generator. The first basic type comprises a group of devices which utilize the reinforcement of waves. The second basic type utilizes a length of transmission medium with the signal being removed at taps separated by equal lengths of the transmission medium. The removed signals may be weighted and summed to provide the desired system output. The third basic type of tapped delay line recirculates the signal to allow the desired fundamental and harmonic frequencies to reinforce themselves.
Acoustic tapped delay devices and coaxial cable delay devices are commonly used in construction of the three basic types of tapped delay lines. Coaxial cable and acoustic devices both have the disadvantages of limited band width, which renders such devices useless both at high carrier frequencies and with short duration signal pulses. At frequencies above 100 mhz, a coaxial cable is subject to losses so severe that high frequencies will not be transmitted accurately. In addition, if the pulse being transmitted is of extremely short duration, such as 1 nanosecond, the pulse will be degraded and will spread out rather than remaining sharply defined. Coaxial cable is also susceptible to electromagnetic interference, particularly when the frequencies being transmitted are relatively high. The main disadvantage of acoustic wave devices is that their upper operational frequency limit is approximately 1 GHz and many applications necessitate word generation systems which are operable at higher frequencies.
The use of an optical fiber tapped delay line in the construction of a word generator has significant advantages in that the optical fiber tapped delay line is not susceptible to electromagnetic interference, is operable at relatively high frequencies, and is substantially less bulky than coaxial cable.
Therefore, there is a need for a fiber optic digital word generator comprising a number of taps capable of removing a signal from a tapped delay line and further comprising means for weighting the outputs to produce a predetermined digital word.